Those that are not studied are one thing, and those that are rare are another. Even if a school is not represented in a guild above level 3, it is still more useful. But taking away the Darkness from humans is essentially depriving them of a powerful and in-demand weapon. If Darkness is preserved for humans, then reducing the chance for non-core schools by 2% can still be accepted. But under no circumstances should the chances of this abomination be increased.
Added 1 hour 25 minutes ago
Appealing to people is not relevant; light and darkness are what make up the nature of humans and are reflected in the gameplay. Summoning, on the other hand, is a school for magical races, implying high magic and knowledge, wizardry, and education. Again, no one forces you to play a dark knight; the choice is yours. But unlike reducing or summoning, it is impossible to call darkness useless for humans.
I wonder, would you like it if humans were deprived of light? After all, it is quite realistic - besides, humans benefit more from darkness - teleport, mass haste, and stone skin can also be obtained from skills, but the debuffs from darkness cannot be replaced by anything.
I agree that darkness is very useful for them, and often even more useful than light, but we should not base our decision on the fact that "it is very useful for them" - darkness and Air Magic would be great - a rain of enchanted arrows with berserk, and that's it, game over. In this case, I am guided by the same logic as the developers of Heroes 7 (they don't have darkness in the human guild) - in the cities of Erathia, there should be no magic of Malassa; humans are zealous fanatics of the school of light, darkness is unacceptable for them, only light. The question remains with the guild - as I understand it, it will not be possible to make only a column with light magic in the guild, then something from summoning or chaos is needed - darkness is contrary to the teachings of Erathia, but summoning and chaos are neutral, hence the choice between them as a second school. Of course, it is possible to leave the possibility of developing darkness for humans, but it must be removed from the city (let's say that we will leave a chance for humans to have a secret passion for darkness, but officially we have nothing to do with darkness :D)
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...